The rapid growth of the number of connected devices and supported cloud services is driving the introduction of beyond-100 Gbit/s optical channels in the DWDM optical transport infrastructure: 400 Gbit/s will be the first step, followed by 1 Tbit/s. Due to a) the difficulty of providing electronic circuits to run at such high speeds and b) performance degradation introduced by linear and non-linear impairments in optical fiber, 400 Gbit/s and 1 Tbit/s transmissions typically rely on multi carrier techniques: in the most general scheme M tributary data flows are mapped into N independently modulated optical carriers where overhead for FEC or pilot symbols is added to payload bits. These data flows with FEC overhead are used to modulate carriers and converted to optical domain signals to create an optical multicarrier signal. Adaptive modulation format is usually proposed as a technique to adjust the aggregate capacity accordingly to the propagation impairments experienced by the channels so that high order modulation formats, e.g. 16QAM, can be used on short links in order to maximize the spectral efficiency while low order modulation formats (e.g. QPSK) could be configured for longer link or in case of unexpected link degradation.
The deployment of new infrastructures is usually a major source of cost for operators so that 20 or 30 years long network lifetime needs to be planned in order to have a full return of the investment. During this period, the traffic demand is expected to significantly increase so that, from a mere business perspective, increasing the spectral efficiency is just a way to save cost prolonging the infrastructure lifetime. This is the main reason behind the introduction of 40 and 100 Gbit/s optical interfaces and explains why they are being installed despite them never reaching a satisfactory cost point with respect to an equivalent number of 10 Gbit/s optical interfaces. Of course, similar considerations apply for 400 Gbit/s and Tbit/s optical interfaces.
Although very high values of spectral efficiency are possible in principle, technology issues and actual network characteristics, such as type of deployed fiber and presence of amplification noise, limit the values achievable in practice to a few times the spectral efficiency carried by today 50 GHz dense DWDM networks with 100 Gbit/s channels, that is approximately 2 bit/s/Hz. It is also known to provide adaptive super-channels, with optimising of sub-channel bandwidth and spacing according to OSNR.